The present invention relates to the general subject of ballasts for powering gas discharge lamps. More particularly, the present invention relates to a method for starting a discharge lamp.
Discharge lamps, in particular low-pressure discharge lamps, which are termed lamps below for short, frequently have electrodes that are designed as electrode filaments.
The starting of such a lamp is described below. It can be subdivided into 3 phases:
1. Preheatingxe2x80x94for this purpose, the electrode filaments are flowed through by a preheating current and heated up thereby. The electron work function at the electrode filaments is thereby reduced.
2. Ignitionxe2x80x94an igniting voltage that initiates the gas discharge in the lamp is applied to the lamp for this purpose. This may be a DC or AC voltage. A resonant ignition is customary in the case of electronic operating devices for low pressure discharge lamps, for which reason the igniting voltage is mostly an AC voltage in this instance.
3. Operationxe2x80x94an operating current is applied to the lamp after the ignition. The operating current must be an alternating current, since separation of the gas or plasma in the lamp otherwise occurs. In the case of customary low-pressure discharge lamps, positively charged mercury ions play an important role in the generation of light. Direct current would cause the mercury ions to accumulate at the cathode, and light would no longer be generated at the anode. An alternating current with a frequency in the range of from 30 kHz to 150 kHz is applied to the lamp in the case of commercially available electronic operating devices.
The preheating is important for the following reason: if an electrode filament is cold, it forms a high cathode fall for the emission of electrons, that is to say for the case in which it acts as cathode. This cathode fall effects a considerable acceleration of incoming mercury ions. The mercury ions striking the electrode filament with high energy lead to rapid wear of the electrode filament, and thus to a short service life of the lamp.
The preheating therefore constitutes a quality feature for a high-quality electronic operating device. However, the implementation of the preheating in terms of circuitry means a substantial outlay, which constitutes a substantial part of the costs of the operating device. What complicates the implementation of the preheating in terms of circuitry is the fact that the electrode filaments to be heated lie at different ends of the lamp. That is to say, the circuit components for heating the two electrode filaments must be designed such that they allow an igniting voltage to pass to the lamp and also withstand it without being damaged.
It is an object of the present invention to provide a method in which the electrode filaments of a discharge lamp can be preheated easily and cost-effectively.
According to the invention, only one of the two electrode filaments of a lamp are preheated, in order to reduce the outlay on circuitry.
As set forth in the section relating to the prior art, an electrode filament is damaged only when it is cold and momentarily acts as a cathode. In the case of ignition with the aid of an AC voltage, the ignition will take place when that electrode which was preheated is momentarily the cathode, because in this state the voltage required for ignition is at its lowest. It follows that the ignition process itself does not cause any damage to the electrode filament which is not preheated in accordance with the invention. Also possible is an ignition with the aid of DC voltage, in the case of which the non-preheated electrode filament acts as anode. In this case, as well, the ignition process does not cause any damage to the electrode filament that is not preheated in accordance with the invention.
During operation following the ignition, however, a non-preheated electrode filament is always damaged when it is momentarily the cathode. According to the invention, the operation of the lamp is subdivided into two subphases. In the first subphase following the ignition, direct current is applied to the lamp in accordance with the invention, the non-preheated electrode filament acting as anode. Since the anode is not exposed to the bombardment of the mercury ions, it is also not damaged when it has not been preheated. The non-damaging electron bombardment heats up an anode in operation of the lamp. That is to say, the non-preheated electrode filament is heated up in the first subphase of operation. If this electrode filament has reached a temperature that also permits non-damaging operation as cathode, the first subphase of the operation ends in accordance with the invention. In the second subphase of the operation, following thereupon, alternating current is applied to the lamp. The second subphase corresponds to the normal operation of the lamp. The first subphase according to the invention is of such a short duration that the abovementioned separation effects do not occur. After at most 2 seconds, the non-preheated electrode filament operated as anode is at a temperature level that permits damage-free operation as cathode.
As already mentioned, the preheating according to the invention of only one electrode filament leads to a substantial reduction in the outlay on circuitry. Since there is a free choice of the electrode filament that is preheated, that electrode filament is preheated which requires the least outlay on circuitry because of its ground reference. The preheating of the electrode filament without ground reference generally requires a higher outlay on circuitry. This can be eliminated according to the invention.
The advantage of the present invention resides, however, not only in the reduction of the outlay on circuitry, but also in a reduction in the outlay on connecting the lamp. Usually, a lamp with two electrode filaments has four terminals. If both electrode filaments are preheated, it is necessary for four connecting wires to be run to the lamp. However, three connecting wires suffice if only one electrode filament is preheated according to the invention. Only the electrode filament that is preheated is connected at its two terminals. One connecting wire suffices for the non-preheated electrode filament.
The description of the invention is largely restricted to one lamp. However, the idea of the invention can also be extended to starting a plurality of lamps in the way according to the invention.